The effects of β1-adrenergic blockade on cardiovascular oxygen flow in normoxic and hypoxic humans at exercise

exercise steady state, the lower the arterial oxygen saturation ( S aO 2 ), the lower the O 2 return (Qv̅O_2). A linear relationship between these variables was demonstrated. Our conjecture is that this relationship describes a condition of predominant sympathetic activation, from which it is hypothesized that selective β1-adrenergic blockade (BB) would reduce O 2 delivery (QaO_2 ) and Qv̅O_2 . To test this hypothesis, we studied the effects of BB on QaO_2 and Qv̅O_2 in exercising humans in normoxia and hypoxia. O 2 consumption (VO_2 ), cardiac output (Q, CO_2 rebreathing), heart rate, S aO 2 and haemoglobin concentration were measured on six subjects (age 25.5±2.4 years, mass 78.1±9.0 kg) in normoxia and hypoxia (inspired O 2 fraction of 0.11) at rest and steady-state exercises of 50, 100, and 150 W without (C) and with BB with metoprolol. Arterial O 2 concentration ( C aO 2 ), QaO_2, and Qv̅O_2 were then computed. Heart rate, higher in hypoxia than in normoxia, decreased with BB. At each VO_2 , Q was higher in hypoxia than in normoxia. With BB, it decreased during intense exercise in normoxia, at rest, and during light exercise in hypoxia. S aO 2 and C aO 2 were unaffected by BB. The QaO_2 changes under BB were parallel to those in Q. Qv̅O_2 was unaffected by exercise in normoxia. In hypoxia the slope of the relationship between QaO_2 and VO_2 was lower than 1, indicating a reduction of Qv̅O_2 with increasing workload. Qv̅O_2 was a linear function of S aO 2 both in C and in BB. The line for BB was flatter than and below that for C. The resting Qv̅O_2 in normoxia, lower than the corresponding exercise values, lied on the BB line. These results agree with the tested hypothesis. The two observed relationships between Qv̅O_2 and S aO 2 apply to conditions of predominant sympathetic or vagal activation, respectively. Moving from one line to the other implies resetting of the cardiovascular regulation.